Antibody directed against specific receptors for antigenic determinants given to normal animals suppresses the response to the antigen. Animals immunized with an antigen produce not only antibody but can themselves produce antibody against the receptor for that antigen. Anti-receptor antibody given to normal adults produced suppression lasting several weeks, and cells from the suppressed animals respond normally on adoptive transfer or in culture. In contrast, anti- receptor antibody given to neonates produces suppression lasting many months, and suppression is apparently due to depletion of the responding clone of cells since cells from such suppressed animals do not respond on adoptive transfer or in culture. For these findings, we use 2 models: the response of the BALB/c mouse to phosphorylcholine, and the response of the Lewis rat to Brown Norway alloantigens. In the next phase of our work we will study the mechanism whereby anti-receptor antibody causes suppression in adult and neonates.